A packaging technology referred to as Through Mold Interconnect (TMI) is commonly used in smaller (e.g., mobile) devices to effect tight integration of two separately packaged die into another, larger overall package.
FIG. 1 shows an example of a TMI structure in a package-on-package (PoP) structure. Here, a first packaged die 101 is electrically connected to the substrate 102 of the larger package 103 through coalesced solder balls 104 within respective vias 105 of an encapsulation epoxy mold compound 106. A second semiconductor die 107 resides within the mold compound 106, and, as a consequence, the second semiconductor die 107 is also regarded as “packaged”. The overall structure therefore tightly integrates a first packaged die just above a second packaged die.
In a common application, the first packaged die 101 is a memory chip and the bottom semiconductor die 107 is a System-on-Chip (SoC) having one or more processing cores, a memory controller and various I/O units such as a wireless interface unit and a display interface unit. The memory chip is electrically coupled to the memory interface of the SoC's memory controller through the coalesced solder balls 104 and electrical traces within the substrate 102 that are coupled to lands 109 that connect to the memory interface I/Os.
Power and ground are also supplied to the packaged memory chip 101 through other coalesced solder balls 104 that are coupled by traces through lower substrate 102 to the solder balls 110 of the overall package 103. Signaling between the SoC and the system outside the overall package 103 (e.g., signaling to/from a display, signaling to/from wireless antennae circuitry) are carried by traces within the lower substrate 102 between corresponding lands 109 and solder balls 110.
FIGS. 2a and 2b show a prior art method of attaching the first packaged die 201 to the lower substrate 202 (for ease of drawing purposes, various details depicted in FIG. 1 have been omitted from the remaining drawings). As observed in FIG. 2a, the lower substrate 202 as originally manufactured includes lower solder balls 220. The first packaged die 201 likewise includes upper solder balls 221. The upper solder balls 221 of the first packaged die 201 include flux 223 to promote wetting and coalescing of solder balls 220, 221 during attachment of the first packaged die 201 to the lower substrate 202. In order to attach the first packaged die 201 to the lower substrate 202, the first packaged die 201 is initially oriented above the lower substrate 202 such that upper solder balls 220 are aligned above the lower solder balls 221.
As observed in FIG. 2b, the first packaged die 101 is then lowered such that, ideally, solder balls 220 make proximate contact with solder balls 221 with flux 223 between them. A high temperature is applied to reflow solder balls 220, 221 together with flux 223 acting as a promoter of the coalescing. After the reflow and removal of the higher temperature, the solder balls 220, 221 are coalesced to form coalesced solder balls.
FIGS. 3a and 3b depict a problem that has been encountered with the process of FIGS. 2a and 2b (note that the more sophisticated package of FIGS. 3a and 3b includes more than one row of solder ball pairs along the package edge). As observed in FIG. 3a, owing to any/all of solder ball pitch tolerances, solder ball shape differences/imperfections, differences/imperfections in the flatness of the top surface of mold compound 306, imperfections in the flatness of the underside of first packaged die 301, etc. the aforementioned “contact” between solder balls 220 and 221 as depicted in FIG. 2b does not occur across 100% of the solder ball contact pairs. Instead, as depicted in FIG. 3a, some solder ball pairs 320_1, 321_1 will make proper contact while other solder ball pairs 320_2, 321_2 will not make proper contact (or any contact)—at least during initial placement of the first packaged semiconductor die 301 on the lower substrate 302.
During the high temperature reflow, as observed in FIG. 3b, owing at least in part to the softening and deformation of the solder ball pairs 320_1, 320_1 that are in contact, the first packaged semiconductor die 101 typically compresses closer to the lower substrate 102 which can have the effect of causing solder pairs 320_2, 321_2 that were not in contact with one another to finally make contact. Nevertheless, the original lack of contact and/or contamination on solder balls 320_2, 321_2 at the start of the reflow process can cause insufficient wetting of the lower solder ball 321_2 by the flux 323 that was formed on the upper solder ball 320_2.
Specifically, if too much time elapses during the reflow process before the non contacting solder ball pairs 320_2, 321_2 finally make contact with one another, the flux 323 on the upper solder ball 320_2 will compositionally degrade (owing to the higher reflow temperatures). As such, by the time contact is finally made, the flux 323 is no longer capable of properly cleaning the lower solder ball 321_2. The two solder balls 321_2, 322_2 therefore do not coalesce resulting in a bad electrical and physical connection.
Additionally, as observed in FIG. 3b, when the first packaged semiconductor die 301 compresses lower toward the lower substrate 302, the bottom of the first packaged die 301 can make contact with the outer edges 325 of the mold compound 306 which can “close” or otherwise “seal off” previously existing openings 326 (refer to FIG. 3a) that existed between the first packaged semiconductor die 301 and the mold compound 306. These openings 326 permitted the applied heat to easily reach and soften the solder balls 320, 321. After the softening and deformation of the solder balls that were in contact 320_1, 321_1, however, the collapse of the first semiconductor package 301 onto the mold compound 306 and the sealing off of the openings 326 likewise seals off the pathways for the applied heat to reach the solder balls 320, 321. As a consequence, less heat begins to be applied to the solder balls 320_2, 321_2 that have only just came into contact. The application of less heat to solder balls 320_2, 321_2 is believed to further exacerbate the problem of successfully coalescing them.